1. Field of the Invention
The present invention relates to a semiconductor circuit comprising a plurality of thin film transistors (abbreviated hereinafter as "TFTs"), and also to a process for fabricating the same. According to the fabrication process of the present invention, the TFTs are formed either on an insulator substrate such as a glass substrate or on a semiconductor substrate such as a single crystal silicon substrate. More particularly, the present invention relates to a semiconductor circuit comprising a low-speed operating matrix circuit, for example, a circuit suitable for use in liquid crystal displays, and a high-speed operating peripheral circuit for driving the matrix circuit.
2. Prior Art
Active study is now being made on semiconductor devices of insulated-gate type comprising an insulator substrate having thereon a thin film active layer (which is sometimes referred to as "active region"). In particular, much effort is paid on the study of insulated-gate transistors of thin film type, i.e., the so-called thin film transistors (TFTs). The TFTs are formed on a transparent insulator substrate, so that they can be employed mainly for controlling each of the pixels or in driver circuits of matrix-driven display devices. The TFTs can be classified into, for example, amorphous silicon TFTs and crystalline silicon TFTs, according to the material and the state of the semiconductor employed in the TFT.
In general, semiconductors in an amorphous state have a low electric field mobility. Accordingly, they cannot be used in TFTs in which high speed operation is required. The present day research and development is therefore devoted to crystalline silicon TFTs for use in the fabrication of high performance circuits.
In contrast to the amorphous semiconductors, crystalline semiconductors have higher electric field mobilities, and are therefore suitable for use in the high speed operation of TFTs. Crystalline silicon is further advantageous in that a CMOS circuit can be easily fabricated therefrom, because not only an NMOS TFT but also a PMOS TFT is available from crystalline silicon. Accordingly, there is proposed an active-matrix driven liquid crystal display having a so-called monolithic structure comprising crystalline TFTs in CMOS, not only in the active matrix portion but also in the peripheral circuit (such as the driver circuit) thereof.
FIG. 3 is the block diagram of a monolithic active matrix circuit for use in a liquid crystal display. Referring to FIG. 3, a peripheral driver circuit comprising a column decoder 1 and a row decoder 2 is provided on a substrate 7. Pixel circuits 4 each comprising a transistor and a capacitor are formed in a matrix region 3. The matrix region is connected to the peripheral circuit by interconnections 5 and 8. A high speed operation is required to the TFTs in the peripheral circuit, while the TFTs in the pixel circuit are required to have low leak current. These two characteristics are indeed in conflict with each other, but still, the two TFTs had to be formed on the same substrate.
However, the TFTs fabricated by a same process exhibit similar characteristics. For instance, crystalline silicon can be obtained by laser annealing, i.e., by a crystallization using laser. However, if the same crystalline silicon obtained by laser crystallization is used in a TFT for a matrix region and in that for a peripheral driver circuit, the both TFTs yield similar properties. One may think therefore of crystallizing the silicon for the TFT of the matrix region by thermal crystallization, and employing laser crystallization for that of the peripheral driver circuit. However, thermal crystallization requires performing annealing at 600.degree. C. for a duration as long as 24 hours or even longer, or at a temperature as high as 1,000.degree. C. or higher. The former treatment considerably lowers the throughput, and the latter process cannot be applied to cases in which substrates other than those made of quartz is used.
The present invention aims to overcome the aforementioned difficulties in the fabrication of a semiconductor circuit. However, the process should not be a complicated one nor one which accompanies low product yield or an increase in product cost. An object of the present invention is to easily provide two types of TFTs, i.e., a TFT having high mobility and a TFT having a low leak current, by incorporating minimum process modifications. In this manner, two types of TFTs can be readily fabricated by a process suitable for mass production.